A conventional surface light source device is illustrated in FIGS. 1 and 2. FIG. 1 is an exploded perspective view, and FIG. 2 is a cross-sectional view. A surface light source device 1 comprises an optical (wave)guide plate 2 for confining and propagating light, a light emitting device 3, and a reflecting plate 4. The optical guide plate 2 is formed of resin which is transparent and has a high index of refraction, for example, polycarbonate resin or methacrylic resin, and a diffuse pattern 5 is formed by irregular processing, dot printing of diffuse reflective ink or the like on the lower surface of the optical guide plate 2. The light emitting device 3 is one having a plurality of so-called point light sources 7, for example, light emitting diodes (LED) mounted on a circuit board 6, and is opposite to an end surface (a light incidence surface 8) of the optical guide plate 2. The reflecting plate 4 is formed of a white resin sheet, for example, which has a high index of reflection, and is affixed to the lower surface of the optical guide plate 2 on both its sides by double-sided tapes 9.
Light emitted from the light emitting device 3 and introduced into the optical guide plate 2 from the light incidence surface 8 is confined in the optical guide plate 2 and travels by being totally reflected from the inside of the optical guide plate 2. The light inside the optical guide plate 2 is diffuse reflected upon impinging on the diffuse pattern 5. A light beam f1, incident on a light output surface 10 at a smaller angle than the critical angle in the total reflection, of the reflected light beams is emitted outward from the light output surface 10. A light beam f2 passing through a portion, where the diffuse pattern 5 does not exist, on the lower surface of the optical guide plate 2 is returned to the optical guide plate 2 again upon being reflected from the reflecting plate 4, so that the loss of the quantity of light on the lower surface of the optical guide plate 2 is prevented from occurring.
In the conventional surface light source device 1, however, a light beam f3, entering a portion between the lower surface of the optical guide plate 2 and the reflecting plate 4, of the light beams emitted from the light emitting device 3 enters the optical guide plate 2 from the lower surface of the optical guide plate 2 upon being reflected from the reflecting plate 4, and is emitted from the light output surface 10 of the optical guide plate 2 without being totally reflected, as shown in FIG. 3. Therefore, the intensity of the output light is large in the vicinity of the light incidence surface 8, as shown in a graph of light output intensity characteristics on the light output surface 10 in FIG. 4. As a result, the luminance of the emitted light is high in the vicinity of the light incidence surface 8 (an area where the luminance is high is indicated by reference numeral 11), and the degree of non-uniformity of the luminance distribution on the light output surface 10 of the optical guide plate 2 is high.
In the surface light source device 1, a linear light source such as a cold-cathode ray tube or a hot-cathode ray tube is replaced with the point light sources 7 such as light emitting diodes in order to reduce the power consumption. The point light sources 7 such as light emitting diodes are disposed in line and brought into a pseudo linear light source. Used is the optical guide plate 2 which is fabricated in accordance with the same design philosophy as that in a case where the linear light source is used. Particularly, the diffuse pattern 5 on the optical guide plate 2 is the same as that in the case where the linear light source is used. That is, reflecting elements 5a constituting the diffuse pattern 5 are disposed in a direction parallel to the light incidence surface 8, as shown in FIG. 6. As the distance from the light incidence surface 8 increases, the density of the reflecting elements gradually increases. Since the optical guide plate 2 suitable for the point light sources 7 is not used, the luminance distribution is non-uniform not only in a direction along the length of the optical guide plate 2 (the direction in which light travels in the optical guide plate 2 upon being totally reflected) but also a direction along the width thereof as shown in FIG. 7 (a direction perpendicular to the direction along the length, that is, an X--X direction shown in FIG. 6).
In the conventional surface light source device 1, a light beam f4, reaching an end surface opposite to the light incidence surface 8 and both side surfaces thereof, of the light beams propagating while being confined inside the optical guide plate 2 leaks outward from the end surface or the side surfaces, so that the utilization efficiency of the light is reduced, and the luminance of the surface light source device 1 is particularly decreased at edges of the light output surface 10.
Furthermore, the diffuse pattern 5 is designed in accordance with the same philosophy as that in the optical guide plate for the linear light source, as described above, in the optical guide plate 2 used in the conventional surface light source device 1, and all the reflecting elements 5a are disposed with they being directed in the same direction. Since the directionality of the diffuse pattern 5 is not designed such that the best light output efficiency is obtained with respect to the point light sources 7, so that the output efficiency of the optical guide plate 2 is low, and the luminance of the surface light source device 1 is decreased.